Polymer foams, such as polyisocyanurate and polyurethane foams have not previously been able to be produced using an extruder, as the foaming material "firms" in the extruder and blocks the equipment.
Richie, U.S. Pat. No. 3,466,705, describes apparatus for extruding foamable thermoplastic material, such as polystyrene, preferably in tubular form. Richie discloses that the apparatus can be used for thermosetting materials, such as polyurethanes, but that steam or hot water would have a damaging effect and therefore are not used. A foam-augmenting gas such as freon is used as the foaming agent. Richie uses an expansion space of transverse cross-sectional area which increases in size gradually in the direction of travel of the extruded product.
The disadvantages of the prior art are overcome by the invention, as described below.